﻿of the Republic of Salvador^ Central America. 29 



of the larger hornblendes are often filled with grains of pyrox- 

 ene. The pyroxene, which is less abundant than hornblende, 

 consists of a pleochroic, orthorhombic and a green, monoclinic 

 variety, the former being in excess. Magnetite appears as a 

 regular porphyritic constituent. 



The groundmass of the pumiceous varieties is a colorless 

 glass with a few microlites. A noncrystalline form was col- 

 lected at San Francisco Mercedes. 



The most notable occurrence of hornblende-pyroxene-andesite 

 is the rock of the new volcano which first made its appearance 

 above the water in the center of Lake Ilopangoon the morning 

 of January 21, 1880. Lake Ilopango measures between nine 

 and ten miles in length, is very irregular in width, presenting 

 about forty square miles of surface. It lies about five miles 

 due east of the City of San Salvador and about 1500 feet above 

 sea-level. Mr. Goodyear, who had been studying the very re- 

 markable series of earthquakes which occurred all over Sal- 

 vador in December of the preceding year, recorded over six 

 hundred earthquake shocks, all of them sufficiently severe 

 to be perceptible without the use of instruments. On the day 

 following the first eruption Mr. Goodyear* visited the young 

 volcano and brought away with him a piece of ihe rock still 

 hot from the steam issuing from the numerous vents. 



The breaking out of this volcano was generally regarded 

 by the people of San Salvador as having saved the city from a 

 serious calamity. 



The rock is a gray pumiceous glass, full of brilliant feld- 

 spars, 4 or 5 mm and less in length, and numerous small crystals 

 of hornblende with hypersthene and augite, and occasionally a 

 grain of olivine. It is worthy of mention that within a short 

 distance from this outburst of hornblende-pyroxene-andesite, 

 and within twelve months of the time, the volcano Izalco 

 threw out basalt from the crater. 



Hornblende- Mica- Andesite. — This type is poorly shown in the 

 collection, only three localities in the Republic being repre- 

 sented. They carry beside the porphyritic crystals of plagio- 

 clase, hornblende, hypersthene, and a little augite, variable 

 amounts of brown mica and quartz, and in this way form a 

 transition to the dacites. They resemble some varieties of the 

 hornblende-mica-andesites found at "Wc-shoe, Nevada. 



Dacite. — Rocks of this type are found in numerous localities 

 throughout Salvador, and present quite a variety of forms. 

 They are mostly light colored in shades of gray and purple, 

 and frequently unite characters both of andesite and rhyolite. 



*W. A. G-oodyear: Earthquake and Volcanic Phenomena, December, 1819 

 and January 1880, in the republic of Salvador. Panama, 1880. 



